Dual Nature of Magnetism in a Uranium Heavy-Fermion System

The duality between the localized and itinerant nature of magnetism in $5f$-electron systems has been a long-standing puzzle. Here, we report inelastic neutron scattering measurements, which reveal both local and itinerant aspects of magnetism in a single-crystalline system of ${\mathrm{UPt}}_2{\mathrm{Si}}_2$. In the antiferromagnetic state, we observe a broad continuum of diffuse magnetic scattering with a resonancelike gap of $\approx 7\mathrm{meV}$ and the surprising absence of coherent spin waves, suggestive of itinerant magnetism. While the gap closes above the Néel temperature, strong dynamic spin correlations persist to a high temperature. Nevertheless, the size and temperature dependence of the total magnetic spectral weight can be well described by a local moment with $J=4$. Furthermore, polarized neutron measurements reveal that the magnetic fluctuations are mostly transverse, with little or none of the longitudinal component expected for itinerant moments. These results suggest that a dual description of local and itinerant magnetism is required to understand ${\mathrm{UPt}}_2{\mathrm{Si}}_2$ and, by extension, other $5f$ systems, in general.

Figure 1

The imaginary part of the dynamical magnetic susceptibility, ${\chi }^{\prime \prime }(\mathbf{Q},\omega )$, corrected for the ${\mathrm{U}}^{4+}$ magnetic form factor, at 5 [(a),(b)] and 50 K [(c),(d)]. The measurement was performed at SEQUOIA, with incident energy $E_i=50\mathrm{meV}$.

Figure 2

(a), (b) ${\chi }^{\prime \prime }(\mathbf{Q},\omega )$ observed at 50 K with $E_i=100\mathrm{meV}$ along the $H$ and $L$ direction, respectively. (c), (d) Fit to the SCR or, equivalently, RPA theoretical model.

Figure 3

Evolution of the generalized susceptibility across $T_N$. (a) The detailed temperature dependence of low-energy spin excitation at $\mathbf{Q}=(1,0,0)$ measured using the HB1 triple axis spectrometer. The counting time for each data point was about 5 min. (b) The autocorrelation function $S(E)$ at 5 (black square) and 50 K (red diamond), obtained from the $Q$-integrated spectral weight of the TOF data. Solid lines show a fit to a model function consisting of a Lorentzian centered at $E=0$ (quasielastic) and a damped harmonic oscillator (inelastic).

Figure 4

Polarized neutron measurement at constant $E=12\mathrm{meV}$ at $T=5\mathrm{K}$. (a) Magnetic scattering ($=$ $\mathrm{HF}\text{−}\mathrm{SF}-\mathrm{HF}\text{−}\mathrm{NSF}$ in Table 2). (b) Transverse spin fluctuation $S_b$ ($=\mathrm{HF}\text{−}\mathrm{SF}-\mathrm{VF}\text{−}\mathrm{SF}$ or $\mathrm{VF}\text{−}\mathrm{NSF}-\mathrm{HF}\text{−}\mathrm{NSF}$) and estimated longitudinal spin fluctuation $S_c$ ($=\mathrm{HF}\text{−}\mathrm{SF}-\mathrm{VF}\text{−}\mathrm{NSF}$ or $\mathrm{VF}\text{−}\mathrm{SF}-\mathrm{HF}\text{−}\mathrm{NSF}$). Each data point has been counted for about 3 hr.